Method for coating bodies with carbon



Patented Aug. 31, 1943 2,328,422 METHOD FOR COATING BODIES WITH CARBON Carl J. Christensen, Flushing,-

N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application Feb ruary 8, Divided and this application October 1940, Serial No.

9, 1940, Serial No. 360,419

Claims.

This intention relates to carbon coating and more particularly to a method for coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas. This application is a division of my application Serial No, 317,834 filed February 8, 1940 for Coating apparatus, now Patent No. 2,285,017, issued June 2, 1942.

It is well known to deposit a coating of carbon on ceramic bodies by heating the bodies to the required temperature and subjecting them to a stream of a hydrocarbon gas so that the gas in contacting with the heated bodies is decomposed and the resulting carbon is deposited on the bodies as a coating of a thickness depending primarily upon the duration of the treatment, the concentration of the hydrocarbon gas and the temperature. Various types of apparatus have been proposed for coating ceramic bodies either employing a batch method or a continuous method when the shape of the bodies to be coated is such as to permit the use of a continuous method. However, in order to obtain a free from soot, it considered necessary to keep the bodies in a stream of a neutral gas during the treatment carbon yielding gas.

. It is anotherobject of the invention to apply such a coating on ceramic tubes or rods by coatingto the use of guiding cosity through which and hence the bodies are a deposit of soot. In order to avoid sooting it is not necessary that the ceramics entering or leaving the furnace be in a neutral gas but only that the gas surrounding the bodies after they leave the hot zone be at a lower temperature than the bodies which it contacts.

A more uniform coating than has been-obtained heretofore results from passing the ceramic bodies through the hot zone of the furnace in a vertical column and without resorting members. Due to the temperature gradient convection currents exist in around the cir- If the ceramic coltroduction of soot.

The various features of the invention may be more clearly understood by reference to the accompanying drawing in which:

Fig. 1 is an elevation view showing one embodiment for realizing the objects of the invention;

Fig. 2 is an enlarged view showing more clearly the direction of flow of the hydrocarbon gas in the preferred arrangement;

Fig. 3 is an enlarged view showing the direction of flow of the hydrocarbon gas in a modifled arrangement; and

Figs. 4 and 5 are views showing spacing elements for maintaining the ceramics in position as they pass through the furnace.

Referring to the drawing, an electric furnace 5 provided with suitable heating elements 6-6 is mounted in a vertical position on a supporting framework 1. Extending through the furnace in a vertical direction is a tubular member 8 of ceramic material, such as quartz or sillimanite. The upper end of the member 8 is joined to the funnel-shaped end 9 of discharge tube N, a suitpartly in cross-section ii being provided to insure a gas-tight sea concentrically positioned within the lower end of member 8 is a sheath tube H which extends to approximately the mid-point of the furnace. The inner diameter of this sheath tube, which is preferably of quartz or similar ceramic material, is somewhat larger than the outer diameter of the ceramic bodies It which are to be coated, while the outer diameter of this tube is less than the inner diameter of tubular member 8 to permit the free flow of gas ,therebetween. The lower end of sheath tube l2 terminates in a hollow T-member l4 into which the hydrocarbon gas enters through :inlet pipe i5 provided with a suitable valve it. A second inlet pipe i1 provided with a control valve I8 is provided at the upper end of member Outlet pipes I9--l9 are provided as shown for discharging the stream of gas leaving the furnace between tubes l2 and 8. Packing material tilt is provided to insure a gas-tight seal. A gland El, preferably of soft rubber, is provided at the lower end of sheath tube E2 to insure a substantially gas-tight seal with the column of ceramics as they enter the furnace. A similar gland 2% is provided at the upper end of the furnace to prevent the escape of gas with the ceramics as they pass out of the furnace.

For conveying the ceramic bodies through the furnace at a uniform and predetermined rate, a mechanism is provided consisting of a fixed grooved pulley 23 driven by means of a motor 25 through a suitable speed-reducing means and a grooved pulley 26 which is pressed against the ceramic bodies by spring mechanism 2i.

In order to maintain the ceramic bodies, as they pass through the furnace, in a fixed vertical column, spacing members 2%! are provided hav-= ing an outer diameter substantially that of the bodies to be coated. As a rule, the ceramics are tubular in shape and coated only on the outer surface, in which case a ceramic spacer is employed as illustrated in Fig. 4, having a central. portion 29 substantially the outer diameter of the ceramic to be coated and end portions tt-3@ of suitable diameter to fit within the bores of the ceramics. In case the ceramic bodies comprise solid rods, a spacer may be employed of the type shown in Fig. 5. This spacer is provided. with a central portion 3i substantially the same diameter as the ceramics to be coated and projecting portions 32-42 which are adapted to fit in corresponding openings in the ceramics.

In the operation of this apparatus, the ceramics to be coated are provided with suitable spacers and propelled by means of pulleys 23, 2d at a predetermined uniform speed through gland 2i and sheathtube l2 to the central portion or hot zone of furnace 5 where decomposition of the hydrocarbon gas takes place. The hydrocarbon gas enters the inlet pipe l5 and surrounds the ceramics as they pass upward through sheath tube l2. While this gas may consist of a mixture of a neutral gas with a carbon bearing gas, such as carbon monoxide, petroleum ether,-benzene, etc., the most satisfactory results have been obtained with a mixture of nitrogen and pure methane, the commercial methane being purified by a process such as that disclosed in my copending application, Serial No. 245,340, filed December 13, 1938. As the refractories pass upward through the furnace in a continuous stream, they leave the hot zone and are discharged through tube i0 and gland 22, which prevents an appreciable escape or outward flow of gas. Preferably, valve able packing material operating conditions l8 of inlet tube i1 is closed so that the flow of gas upon leaving sheath tube l2 follows, in general. the direction of the arrows as shown in Fig. 2, there being no appreciableflow of gas in the upper portion of tube 8..

The hot zone of the furnace is preferably maintained at a temperature of from 900 C. to 1200 C. The pure methane does not decompose until the higher temperatures of the furnace are reached, and the appearance of soot is delayed until after a chemical induction period of finite time so that no soot is deposited on the ceramics while they are being conducted to the hot zone of the furnace. At the hot zone the ceramics receive their coating of hard carbon, then pass on to the cooler exit regions of the furnace. This movement of the ceramics from the hot zone of the furnace toward the cold outlet gland 22 causes the coated-ceramics to be hotter than the contiguous gas, thus creating a temperature gradient between the coated ceramic tubes and the furnace tube 8. If the furnace tube 8% is not too small in diameter, this gives rise to a conduction zone around the coated ceramics in theirprogress toward the outlet which protects them from the deposition of soot.

In case a greater temperature gradient should be found desirable to meet certain conditions, this may be brought about by also passing hydrocarbon gas into the furnace through auxiliary inlet pipe ii and valve iii. This stream of cold gas will prevent the accumulation of a body of stagnant heated gas that would otherwise form in the upper portion of the furnace, and thus increases the temperature gradient. Under these the direction of flow of hydrocarbon gas in the furnace will be in the directions indicated by the arrows in Fig. 3.

While it would be ing gas from the furnace by means of vents at the center of the hot zone, such a construction would be more complicated than the proposed method in which space is provided between tubes 8 and 02 for the removal of the gas from the hot zone.

Following the proposed method, either with or without passing a stream of hydrocarbon gas counter to the movement of the ceramics, it has been found possible to provide the ceramics with uniform coatings of hard carbon entirely devoid of soot without resorting to the use of neutral gases or unnecessary complications in the construction of the furnace.

What is claimed is:

l. The method of coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas which consists in conveying said bodies into a furnace, causing unheated flow of the hydrocarbon gas uniformly over the entire outer surfaces of said bodies as they enter said furnace, maintaining the temperature of the oven such as to cause thermal decomposition of the hydrocarbon gas and removing the carbon-coated bodies from the hot zone and out of the furnace while maintaining the bodies at a distinctly higher temperature than the contiguous gas so that an appreciable temperature gradient exists in the gas between the entire outer surfaces of said body and the furnace wall.

2. The method of coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas which consists in conveying said bodies through a furnace while maintained in the form of a vertical column, causing a stream of hydrocarbon gas to enter the furnace in contact with possible to remove the coatto the direction said bodies and pass in the same direction there with to the hot zone of said furnace, removing saidgas from the furnace in a direction opposite of movement of said bodies, and maintaining the central portion of said furnace at a suitable temperature to cause thermal decomposition of the hydrocarbon gas.

3. The method of coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas which consists in continuously conveying said bodies upward said furnace while maintained in the form of a self-sustained column, causing a stream of hydrocarbon gas to enter the furnace in contact with said bodies and pass in the same direction therewith to the hot zone of said furnace, removing said gas from the furnace in the opposite direction, and maintaining the central portion of the furnace at a suitable temperature to cause thermal decomposition of the hydrocarbon gas.

4. The method of coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas which consists in conveying said bodies through a furnace in the form of a vertical column. causing a stream of hydrocarbon gas to enter the furnace in contact with said bodies and ass in the same direction therewith to the hot zone of said furnace, causing a second stream of and in a vertical direction through a hydrocarbon gas to enter the furnace in contact with said bodies as they are discharged and to flow through the hot zone of said furnace in a direction opposite to the direction of movement of said bodies, removing said streams of gas from the furnace in the direction opposite to the direction of movement of said bodies, and maintaining the central portion of said furnace at a suitable temperature to cause thermal decomposition of the hydrocarbon gas.

5. The method of coating bodies with a layer of hard carbon by deposition from a hydrocarbon gas which consists in conveying said bodies through a furnace, causing a stream of hydrocarbon gasto enter the furnace in contact with said bodies and pass in the same direction therewith to the hot zone of said furnace, removing said gas from the furnace in the opposite direction, maintaining the central portion of said furnace at a suitable temperature to cause thermal decomposition of the hydrocarbon gas, and

maintaining a temperature gradient between the entire outer surfaces of said bodies and the walls of said furnace. such as to produce a conduction zone surrounding said bodies as they leave the central portion of said furnace and through whichsoot particles do not penetrate.

CARL J. CHRISTENSEN. 

